1. Field of the Invention
This invention relates to theft detection apparatus of the type in which a protected object is detected by monitoring the magnetic fields produced by responder targets on protected articles when such articles are carried through an interrogation zone in which an alternating magnetic field is generated. In particular the invention comprises improvements relating to the responder targets and their manufacture.
2. Description of the Prior Art
French Pat. No. 763,681 to Pierre Arthur Picard discloses an article theft detection apparatus of the type to which this invention applies. As described in that patent, articles to be protected from theft are provided with responder targets in the form of thin strips of material having a high magnetic permeability and which are rapidly and repeatedly driven into and out of magnetic saturation in the presence of an alternating magnetic interrogation field. An interrogation antenna is provided at an interrogation zone in a passageway leading to the exit of a store or a protected area in a store; and means are provided to cause the interrogation antenna to generate an alternating magnetic field at a given frequency and at an intensity sufficient to saturate a responder target in the interrogation zone. As a result, the responder target itself produces alternating magnetic fields at frequencies which are harmonics of the given transmitter frequency. A receiver antenna is also provided at the interrogation zone to receive the magnetic fields produced by the responder target. The receiver antenna is connected to a receiver which is tuned to detect signals at one or more of the harmonic frequencies produced by the responder target; and an alarm is connected to the receiver to be activated when such detection takes place.
Various refinements to the basic device shown in French Pat. No. 763,681 are shown and described in pending U.S. application Ser. No. 509,292 filed June 29, 1983 and U.S. Pat. Nos. 4,074,249, 4,118,693, 3,820,103, 3,820,104, 3,673,437, 3,737,735, 3,534,243 and 4,326,198.
The French Pat. No. 763,681 discloses that if the responder target is of elongated configuration, it will be saturated earlier, i.e. at a lower intensity of the interrogating magnetic field, than a short target or responder. U.S. Pat. No. 3,790,945 states that in the case of a responder target with dimensions providing a very high ratio of length to square root of cross-sectional area, the signal resulting from the presence of the very high harmonics is far greater than that resulting from greater amounts of the same materials having non-preferred dimensions. According to this last mentioned patent, the responder target (i.e., "marker") should have a high magnetic permeability (Permalloy metal is suggested); and it should be provided with a very slender cross-section as compared with length, as for example a cross-sectional area of 0.0004 square centimeters, and a length of 4 centimeters or more, this same being comprised in a ribbon not thicker than 0.00125 centimeters. This patent also states that the responder target is preferably provided with a ratio of length to square root of cross-sectional area which exceeds 200.
In the past, responder targets of Permalloy or similar material have been made by rolling the material to a very thin sheet, then heat treating the sheet to obtain the necessary magnetic properties and then slitting the sheet into strips of desired width. Usually the strips are laminated to paper or plastic ribbon as described in U.S. Pat. Nos. 3,820,103 and 4,074,249. Also, in cases where the responder targets are to be capable of being deactivated, a hard magnetic material of high coercive force (i.e. a semi-permanent magnet material) is laminated alongside the Permalloy strip, as described in U.S. Pat. Nos. 3,747,086 and 3,765,007.
While it is desirable to provide responder targets having a high ratio of length to square root of cross sectional area, there are certain practical problems which limit this ratio. Firstly, highly permeable, magnetically soft materials such as Permalloy do not have substantial structural strength; and when they are rolled into very thin sheets, e.g. about 0.00125 centimeters, they are difficult to handle. Secondly, the magnetic properties of these materials is adversely affected by strain. Accordingly when the thin rolled sheets are handled, they tend to stretch and their magnetic characteristics deteriorate. Also, when the sheets are slit into strips, the strain produced along their edges deteriorates their magnetic characteristics so that there is a limit to how narrow the strips may be cut. As a result of these practical problems, the responder strips of the prior art generally had a cross sectional area of about 0.0008 square centimeters and a length of about 7.6 centimeters. These targets were expensive from the standpoint of the cost of materials involved and they required complex and expensive machinery for rolling, slitting and assembly.